Paperboard cartons are widely used for packaging liquid food products, such as milk and fruit juices. It is typical to use resin coatings, films, and foils in one or more layers to provide a barrier between the contents of the container and the paperboard. The containers also are usually provided with exterior resin coatings that provide a water barrier, printable surface and improved container appearance. The exterior resin coatings are used in the formation of overlapping seals required during carton manufacture, set-up and sealing.
Aluminum foil lined paperboard cartons in use today have represented an initial step toward development of lighter, cheaper and more disposable paperboard packages for liquid foods which are oxygen and light sensitive. In such foil cartons, the inner material exposed to the packaged product has historically been polyethylene. It has been used because of its heat sealing capabilities, as well as low cost, organoleptic and governmental regulatory considerations. Unfortunately, however, polyethylene has the property of absorbing ethanol and many other organic chemicals, including flavor oils found in food products such as juices. This absorption property of polyethylene not only causes flavor loss during storage of food products, but can also result in lowered adhesion between the structural layers of the packages, particularly between foil and polyethylene layers. This has been identified even in the packaging and storing of the most common juices.
Current atempts to package wine in foil lined cartons have resulted in inconsistent seals over the required storage lives for the packages, apparently due to degradation of the polyethylene heat seals due to reactions caused by the ethanol content of the wine. These adhesion problems can be avoided by the use of special adhesive resins between layers in the laminate. But use of adhesive layers results in laminates having six or seven layers, which in turn requires special coextrusion equipment or multiple extruder passes to manufacture the laminate. The additional layers also add to material costs in a product area where low unit cost and high production quantities drive the economics of the industry.
A series of new paper substrate laminates have been appearing recently, reflecting current attempts to circumvent the organic absorption problem, simplify production procedures, and lower costs by using fewer layers while still maintaining good oxygen barrier properties. None of the paper laminates developed to this time appear to meet all of the criteria of light, volatile components and oxygen barrier characteristics, light weight, and ease of manufacture required for packaging of juice, wine or other oxygen and light sensitive products. They typically require complicated extrusion equipment and procedures, utilize expensive resins, fail to act effectively as barriers to prevent oxygen or light degradation of food or the loss of critical flavor oils, or fail to be compatible with existing carton manufacturing systems.
This disclosure describes a new paper laminate which improves on previously known laminates in these critical areas, including reduction in the combined laminate weights. Weight is important to shipping costs for the laminate and carton, but even more important to some end use applications, such as flight beverage services for commercial airlines, where reduced weight can be translated into fuel savings.
There have been numerous proposals made in the past for designing such paper substrate laminates to meet the barrier requirements for products susceptible to degradation during storage, such as fruit juices and wine, while also accommodating the physical operating requirements of existing machinery for handling such container blanks.
One example is shown in U.S. Pat. No. 3,927,245 to Roth et al. It describes a laminate in which an ethylene polymer coating is applied to the exterior laminate surface. A second ethylene polymer coating and an olefin polymer coating are coextruded on the inner laminate surface with the ethylene polymer contacting the base paperboard areas. Many variations in such a coextruded laminating coating have been introduced from time to time with varying degrees of success. However, coextruding some multiple resin layers on paperboard requires resin drying equipment and relatively complicated extruding equipment, making quality control difficult. Extrusion coated layers cannot be metallized to provide oxygen and light barrier capabilities.
A subsequent coextruded laminate coating was discussed in U.S. Pat. No. 4,455,184 to Thompson. Polyethylene terephthalate (PET) is utilized with a coextruded layer of polymeric adhesive interpositioned between the polyester resin and the underlying paperboard. While this attempts to gain the known barrier capabilities of polyethylene terephthalate in paperboard laminates for containers, it also encounters the inherent recognized difficulties relating to quality control in such technically complicated extrusion processes and fails to provide a resulting laminate readily adaptable to the flame heat seal seaming processes of common carton blank manufacturing operations.
An additional coextruded coating is shown in U.S. Pat. No. 4,513,036 (Thompson et al.) which teaches coextrusion of polypropylene and a covering coating of low density polyethylene to produce a coating amenable to side-seaming by direct flame systems. Tests of such containers have shown no effective oxygen barrier is exhibited by them. The sandwiching of the polypropylene layer under the polyethylene layer provides little improvement in flavor oil barriers beyond the usual properties of paperboard cartons having low density polyethylene interior coatings.
Another approach to producing such laminates is to laminate a film onto the paperboard layer, which leads to the possibility of metallizing the film to incorporate barrier properties similar to that obtained by metal foil linings. An example of a patent disclosure that teaches the lamination of a polyester film layer for this purpose is U.S. Pat. No. 3,972,467 to Whillock et al. In the disclosure, the laminated polyester film layer is covered by an extruded coating of polyethylene to impart required heat sealing properties to the composite. The resulting cartons exhibit marginal oxygen barrier properties and no improvement in flavor oil barrier properties, since the materials within the container are exposed directly to the polyethylene coating, which is known to exhibit a high degree of absorption.
A laminated paper material that incorporates a coextruded film composed of two or three layer combinations of ethylene-methyl acrylate copolymer and polyester is disclosed in U.S. Pat. No. 4,387,126. It is combined with foil by interposed bonding layers between the barrier and substrate surfaces. Heat sealing is provided by use of an innermost layer of polyolefin film.
Because the industry has recognized the desirable barrier capabilities of polyethylene terephthalate (PET) with respect to its use in the storage of volatile or degradable food products, many researchers and producers of paperboard cartons have attempted to incorporate this material into laminates for such containers. Of principal interest is a commercially introduced laminate having an exterior coating of low density polyethylene, and an inner coating of low density polyethylene sandwiched between the paperboard and an extruded layer of polyethylene terephthalate modified to be heat sealable. Depending on processing, the modification of the polyethylene terephthalate coating may show high levels of stress cracking and has been found to lose much of its moisture barrier properties, in turn requiring the utilization of the coating of low density polyethylene under it. The modification of the PET to make it heat sealable also appears to lower its flavor barrier properties. Because the polyethylene terephthalate is extrusion coated, it cannot be metallized to provide a good oxygen and light barrier within the containers. The multiple extrusion process used to produce this laminate requires complicated extrusion equipment and the handling of molten polyethylene terephthalate, which is recognized as a resin that is difficult to extrude. The unusual difficulties in coating paperboard with this resin cause this laminate to also exhibit inconsistent heat seal properties.
Flexible polyester films are widely used today in the packaging of materials, including food products, within heat-sealed film pouches. Such pouches are typically formed and sealed by pressing the inner surfaces of the sides of the pouch against one another between heated irons. Seals are effected between the abutting inner surfaces. To respond to such packaging needs, the producers of such films have developed coated and coextruded films having different properties at their outer and inner surfaces.
While such films are used in the production of flexible bags and pouches, they have not been found to be readily adaptable to the needs of conventional paperboard container converting equipment. In contrast to the relatively low temperatures and high pressures found in heat sealing applications for bags and pouches, paperboard container blanks are produced by exposing laminated flat blanks to high temperatures in very high speed flame seam sealers that produce lap side seams joining inner and outer surfaces of a flat blank. The temperatures, seaming pressures and speeds encountered in such machinery have posed serious problems in adapting high barrier films, such as polyethylene terephthalate, to the needs of paperboard container manufacturing operations.
The present invention arose inadvertently during tests attempting to use a coextruded oriented PET film as a vapor barrier within a paperboard container. Its discovery was contrary to all indications in published information pertaining to the film, which unequivocally stated that the heat sealable surface of the film would heat seal to a number of different materials, including itself, but not to polyolefins.
The present discovery arose during testing of luminations of flat paperboard blanks having the coextruded, dual-surface polyethylene terephthalate film bonded to both of their surfaces. When flat paperboard blanks formed from this laminate were activated by exposure to a direct flame, it was found that the "self sealable" surfaces of the film layers would not heat seal to one another to produce tubular blanks. A number of adhesive resins were tested on the outside surfaces of the blanks in an attempt to overcome this obstacle. Some did improve the lap seams, but those that showed promise were prohibitively expensive or had other problems.
By accident, laminates including the film and an exterior surface of low density polyethylene coated on the paperboard were tested, again using flame activation techiques followed by heat sealing. It was discovered that these blanks did produce a successful seam. This discovery lead to further testing in conventional flame heat sealing equipment and the development of the present invention. It appeared from the tests that the reason the heat sealable polyethylene terephthalate surfaces would not bond to one another after being activated by exposure to a flame was the surface oxidation that resulted from the high level of flame exposure. The exposure level is much greater than that which results from flame priming, which similarly exposes the film surface to an open flame. The heat sealable polyester will still bond to itself and not to polyethylene after such priming. Apparently oxidation facilitates heat sealing of the outer polyethylene terephthalate surface to a polyolefinic resin, such as polyethylene. Exposure to the flame must "activate" the polyethylene terephthalate and polyethylene surfaces to effect bonding when subjected to a seam sealer. To those familiar with the art, activation is judged adequate when the resin surfaces take on a slightly roughened look, but do not discolor. This activation may be followed rapidly by flame heat sealing, as in a flame lap seam sealer, or may take place as a separate step, followed by non-flame heat sealing.
The products and processes described herein provide a paper substrate-plastic film laminate which offers improved oxygen protection and volatile flavor oil retention at costs significantly less than those now required to produce foil lined cartons. When the film is metallized, the laminate offers moisture, light and oxygen protection similar to that found in foil lined cartons and much better volatile flavor oil retention at a cost still less than that required for foil lined cartons. The laminate presents a high barrier liquid packaging structure which can be produced in a single pass on any tandem extruder equipped with a laminating station. The extruder need only to be able to run the most common thermoplastic resins (typically low density polyethylene) without any additional equipment, such as dryers. The laminate presents the possibility of developing a liquid container suitable for packaging of wine and other oxygen-sensitive products, with a substantial weight reduction in comparison to conventional glass bottles and with even a fifteen percent weight reduction over the foil lined cartons currently in limited use. The package is expected to exhibit improved wine flavor retention, based upon the recognized properties of the resins incorporated within this new laminate.